/**
* Mega pixel image rendering library for iOS6 Safari
*
* Fixes iOS6 Safari's image file rendering issue for large size image (over mega-pixel),
* which causes unexpected subsampling when drawing it in canvas.
* By using this library, you can safely render the image with proper stretching.
*
* Copyright (c) 2012 Shinichi Tomita <shinichi.tomita@gmail.com>
* Released under the MIT license
*/
(function () {

    /**
    * Detect subsampling in loaded image.
    * In iOS, larger images than 2M pixels may be subsampled in rendering.
    */
    function detectSubsampling(img) {
        var iw = img.naturalWidth, ih = img.naturalHeight;
        if (iw * ih > 1024 * 1024) { // subsampling may happen over megapixel image
            var canvas = document.createElement('canvas');
            canvas.width = canvas.height = 1;
            var ctx = canvas.getContext('2d');
            ctx.drawImage(img, -iw + 1, 0);
            // subsampled image becomes half smaller in rendering size.
            // check alpha channel value to confirm image is covering edge pixel or not.
            // if alpha value is 0 image is not covering, hence subsampled.
            return ctx.getImageData(0, 0, 1, 1).data[3] === 0;
        } else {
            return false;
        }
    }

    /**
    * Detecting vertical squash in loaded image.
    * Fixes a bug which squash image vertically while drawing into canvas for some images.
    */
    function detectVerticalSquash(img, iw, ih) {
        var canvas = document.createElement('canvas');
        canvas.width = 1;
        canvas.height = ih;
        var ctx = canvas.getContext('2d');
        ctx.drawImage(img, 0, 0);
        var data = ctx.getImageData(0, 0, 1, ih).data;
        // search image edge pixel position in case it is squashed vertically.
        var sy = 0;
        var ey = ih;
        var py = ih;
        while (py > sy) {
            var alpha = data[(py - 1) * 4 + 3];
            if (alpha === 0) {
                ey = py;
            } else {
                sy = py;
            }
            py = (ey + sy) >> 1;
        }
        var ratio = (py / ih);
        return (ratio === 0) ? 1 : ratio;
    }

    /**
    * Rendering image element (with resizing) and get its data URL
    */
    function renderImageToDataURL(img, options, doSquash) {
        var canvas = document.createElement('canvas');
        renderImageToCanvas(img, canvas, options, doSquash);
        return canvas.toDataURL("image/jpeg", options.quality || 0.8);
    }

    /**
    * Rendering image element (with resizing) into the canvas element
    */
    function renderImageToCanvas(img, canvas, options, doSquash) {
        var iw = img.naturalWidth, ih = img.naturalHeight;
        var width = options.width, height = options.height;
        var ctx = canvas.getContext('2d');
        ctx.save();
        transformCoordinate(canvas, ctx, width, height, options.orientation);
        var subsampled = detectSubsampling(img);
        if (subsampled) {
            iw /= 2;
            ih /= 2;
        }
        var d = 1024; // size of tiling canvas
        var tmpCanvas = document.createElement('canvas');
        tmpCanvas.width = tmpCanvas.height = d;
        var tmpCtx = tmpCanvas.getContext('2d');
        var vertSquashRatio = doSquash ? detectVerticalSquash(img, iw, ih) : 1;
        var dw = Math.ceil(d * width / iw);
        var dh = Math.ceil(d * height / ih / vertSquashRatio);
        var sy = 0;
        var dy = 0;
        while (sy < ih) {
            var sx = 0;
            var dx = 0;
            while (sx < iw) {
                tmpCtx.clearRect(0, 0, d, d);
                tmpCtx.drawImage(img, -sx, -sy);
                ctx.drawImage(tmpCanvas, 0, 0, d, d, dx, dy, dw, dh);
                sx += d;
                dx += dw;
            }
            sy += d;
            dy += dh;
        }
        ctx.restore();
        tmpCanvas = tmpCtx = null;
    }

    /**
    * Transform canvas coordination according to specified frame size and orientation
    * Orientation value is from EXIF tag
    */
    function transformCoordinate(canvas, ctx, width, height, orientation) {
        switch (orientation) {
            case 5:
            case 6:
            case 7:
            case 8:
                canvas.width = height;
                canvas.height = width;
                break;
            default:
                canvas.width = width;
                canvas.height = height;
        }
        switch (orientation) {
            case 2:
                // horizontal flip
                ctx.translate(width, 0);
                ctx.scale(-1, 1);
                break;
            case 3:
                // 180 rotate left
                ctx.translate(width, height);
                ctx.rotate(Math.PI);
                break;
            case 4:
                // vertical flip
                ctx.translate(0, height);
                ctx.scale(1, -1);
                break;
            case 5:
                // vertical flip + 90 rotate right
                ctx.rotate(0.5 * Math.PI);
                ctx.scale(1, -1);
                break;
            case 6:
                // 90 rotate right
                ctx.rotate(0.5 * Math.PI);
                ctx.translate(0, -height);
                break;
            case 7:
                // horizontal flip + 90 rotate right
                ctx.rotate(0.5 * Math.PI);
                ctx.translate(width, -height);
                ctx.scale(-1, 1);
                break;
            case 8:
                // 90 rotate left
                ctx.rotate(-0.5 * Math.PI);
                ctx.translate(-width, 0);
                break;
            default:
                break;
        }
    }


    /**
    * MegaPixImage class
    */
    function MegaPixImage(srcImage) {
        if (window.Blob && srcImage instanceof Blob) {
            var img = new Image();
            var URL = window.URL && window.URL.createObjectURL ? window.URL :
                window.webkitURL && window.webkitURL.createObjectURL ? window.webkitURL :
                null;
            if (!URL) { throw Error("No createObjectURL function found to create blob url"); }
            img.src = URL.createObjectURL(srcImage);
            this.blob = srcImage;
            srcImage = img;
        }
        if (!srcImage.naturalWidth && !srcImage.naturalHeight) {
            var _this = this;
            srcImage.onload = function () {
                var listeners = _this.imageLoadListeners;
                if (listeners) {
                    _this.imageLoadListeners = null;
                    for (var i = 0, len = listeners.length; i < len; i++) {
                        listeners[i]();
                    }
                }
            };
            this.imageLoadListeners = [];
        }
        this.srcImage = srcImage;
    }

    /**
    * Rendering megapix image into specified target element
    */
    MegaPixImage.prototype.render = function (target, options) {
        if (this.imageLoadListeners) {
            var _this = this;
            this.imageLoadListeners.push(function () { _this.render(target, options) });
            return;
        }
        options = options || {};
        var imgWidth = this.srcImage.naturalWidth, imgHeight = this.srcImage.naturalHeight,
        width = options.width, height = options.height,
        maxWidth = options.maxWidth, maxHeight = options.maxHeight,
        doSquash = !this.blob || this.blob.type === 'image/jpeg';
        if (width && !height) {
            height = (imgHeight * width / imgWidth) << 0;
        } else if (height && !width) {
            width = (imgWidth * height / imgHeight) << 0;
        } else {
            width = imgWidth;
            height = imgHeight;
        }
        if (maxWidth && width > maxWidth) {
            width = maxWidth;
            height = (imgHeight * width / imgWidth) << 0;
        }
        if (maxHeight && height > maxHeight) {
            height = maxHeight;
            width = (imgWidth * height / imgHeight) << 0;
        }
        var opt = { width: width, height: height };
        for (var k in options) opt[k] = options[k];

        var tagName = target.tagName.toLowerCase();
        if (tagName === 'img') {
            target.src = renderImageToDataURL(this.srcImage, opt, doSquash);
        } else if (tagName === 'canvas') {
            renderImageToCanvas(this.srcImage, target, opt, doSquash);
        }
        if (typeof this.onrender === 'function') {
            this.onrender(target);
        }
    };

    /**
    * Export class to global
    */
    if (typeof define === 'function' && define.amd) {
        define([], function () { return MegaPixImage; }); // for AMD loader
    } else {
        this.MegaPixImage = MegaPixImage;
    }

})();